1. Field of the Invention
This invention relates to fluid power actuators, and in particular to a dynamic pressure regulator cushion for use with a fluid power actuator.
2. Background of the Invention
Fluid power actuators enjoy considerable popularity in a wide variety of industrial applications, especially in automation and numerical control machines. Either pneumatic or hydraulic fluid may be used to power these actuators. A typical power actuator system comprises a cylinder within which a piston is free to reciprocate. A load is attached to the piston. For simplicity, hereafter the piston/load combination will be most often be referred to as the piston. A pressurized fluid supply is alternately connected to either a first cylinder end or a second cylinder end through a directional control valve. The piston is driven away from the cylinder end to which the pressurized fluid supply is connected. A flow control valve is connected to each cylinder end to control the flow rate of fluid escaping from the cylinder ahead of the piston, which in effect controls the piston speed during most of the stroke.
In operation, the directional control valve permits fluid at driving pressure to flow into a first cylinder end, which drives the piston towards an opposite, second cylinder end. The speed at which the piston travels toward the second cylinder end is controlled by the rate at which fluid is allowed to escape from the second cylinder end through the flow valve associated with the second cylinder end.
The piston rate of speed can be considerable. Thus, it is advisable to provide a cushion to slow the piston before it crashes into the cylinder end wall at the second cylinder end. If the piston were allowed to crash uncushioned into a cylinder end wall, the life of the actuator would be reduced, and metal fragments could even result, causing immediate actuator failure. Therefore, one of the most important components in modem fluid power actuators is the cushion which slows the piston down in a controlled fashion at either extreme of its travel.
Existing Designs
Many conventional fluid power actuators provide a cushion sealing mechanism which blocks the path of fluid out of the cylinder to the flow control valve, forcing the fluid instead to exit the cylinder through a cushion cylinder passage and thence through a cushion valve. This flow restriction (also known as the cushion orifice) causes a rapid increase in retarding pressure (a "pressure spike") ahead of the piston, which acts to decelerate the piston. However, conventional cushion valves are simply another static restriction like that of the flow control valves, so the system will merely tend toward a new equilibrium at a reduced, constant piston speed, until the piston strikes the cylinder end.
The relevant system parameters are depicted graphically in FIG. 9. FIG. 9 shows conventional cushion pressure/piston speed vs. stroke graph 100. Values for supply pressure 106, driving pressure 108, fluid pressure ahead of piston 112, and piston speed 118 are read off ordinate axis 102 for different piston positions during the stroke along abscissa axis 104. The piston stroke starts at one end of the cylinder, at stroke start 101. The piston (and load) accelerates to speed within the cylinder (see piston speed 118), as driven by driving pressure 108. Driving pressure 108 must exceed fluid pressure ahead of piston 112 by an amount sufficient to overcome normal operating speed friction 110.
At cushion start 126 the cushion sealing mechanism blocks the path of fluid out of the cylinder to the flow control valve, forcing the fluid instead to exit the cylinder through a cushion cylinder passage and thence through a cushion valve. This restriction causes pressure spike 114, which causes excessive deceleration 120 in piston speed 118. Because conventional cushion valves are simply another static restriction like that of flow control valves, during cushion stroke 115 the system will merely tend toward a new equilibrium at cushion piston speed 122 (which is less than driving pressure 108 due to cushion stroke friction 116), until the piston strikes the cylinder end at high impact speed 124. The piston stroke ends with the high impact speed 124 collision between the piston and the cylinder end, at stroke end 103.
In order to reduce final high impact speed 124 to an acceptable level, the cushion orifice may be adjusted to a very small opening. This results in an very hard air spring which may cause the piston (and load) to oscillate initially, then travel very slowly over the final portion of the cushion stroke. Neither of these piston/load behaviors is desirable.
One improvement over conventional fluid power actuator cushions is the incorporation of a stepped or tapered cushion sealing mechanism which progressively reduces orifice area throughout cushion stroke for the purpose of maintaining constant cushion pressure as piston speed drops. There are a number of problems associated with this solution. Those cushion sealing mechanisms which do not incorporate seals to separate the stages must rely on very closely held dimensions, which renders them time-consuming to manufacture, and therefore expensive. The cushion sealing mechanism designs which incorporate separate seals for each stage are bulky, and limited in the number of stages they can provide. Designs which incorporate multiple orifices but single seals require that the orifice pass over the seal, risking damage to the seal (see U.S. Pat. No. 5,125,325, granted to Czukkermann). Finally, adjustment for differing loads is difficult.
Especially in air and/or small actuators, the total metering area must be very small to be effective. When distributed over multiple orifices, each one must be extremely small, and thus such cushions are difficult and costly to make, and subject to blockage by contamination. Adjustment for differing loads is difficult with these designs.
U.S. Pat. No. 4,700,611 was granted Kaneko for a complex cushion which required a large piston, multiple seals, and two different valves per cushion. This design suffered from a number of problems. The complex cushions added considerably to the axial dimension of the actuator. Because of its complexity and the number of components, it was costly to make. In addition, the pressure relieving valve controlling the pressure within the cushion had to be set relative to atmospheric pressure, which required a relatively large spring force, which in turn required a relatively large spring. A large spring translates into expense and decreased adjustment sensitivity. Also, the cushioning force of this device was dependent on the pressure applied to the actuator piston when it moved in the opposite direction. A change in line pressure during the time between the charging and cushioning strokes would result in a disproportionate cushioning force.
Another problem associated with the '611 cushion was that its cushioning relied on storage of fluid pressure over the period of time a stroke took place. A small leak could render the cushion ineffective. Still another problem was the requirement for a leak path (drawing item 68) to relieve the pressure remaining in the cushion to allow full holding force at the end of the stroke. If lubricated air were used, this leak path would actually be a constant oil leak to atmosphere. Also, this path remained open during the first part of the power stroke. If for some external reason the piston motion were to be stopped during this first part of the power stroke, the loss of pressurized air through leak path 68 would be continuous.
Finally, this design required a vent to atmosphere in order to function. This vent had to be filtered to prevent contamination, which may require additional plumbing. Also, the vent would exhaust pressurized air to atmosphere, same as leak path 68.